The present invention relates to a covering member of a keyboard and a base plate therefor to which push button elements are set up. The invention relates particularly to a keyboard covering member having push button elements fixed directly to the base plate or having a unit body carrying a push button element fixed thereto. The keyboard is useful for push button input or keyboard input of electric or electronic signals to electric or electronic instruments.
In the prior art, many of electric or electronic instruments are usually combined with a push button input unit or a keyboard input unit composed of a circuit board carrying a plural number of fixed electrode contacts connected by circuit wirings and a covering member having an integrated structure with movable push button elements confronting the fixed electrode contacts. The assembling work in the manufacture of push button input units or keyboard input units involving printed circuit boards is markedly simplified by the use of such covering members. Covering members of keyboards of such a type are usually manufactured by a process including successive steps of putting movable contact elements made of vulcanized electroconductive rubber on the bottoms of each of cavities in the metal mold for the covering member conforming to the arrangement pattern of the fixed contacts of the circuit board and overlaying an unvulcanized insulating rubber base followed by vulcanization of the same under pressure and heating. Accordingly, a separately prepared metal mold is required for each type of the covering members in order to manufacture covering members of keyboards set up with even one or irregularly shaped or arranged push button element. The renewal of the metal mold is quite troublesome involving repeated trials and errors in designing, fabrication, readjustment and inspection with anticipation of the effects of the irregular portions in the covering member on the size change of the whole molded articles during or after molding or in the lapse of time during use. Such a disadvantage is a heavy burden particularly in the case of manufacture of large sized covering members in which matching in the position of the fixed electrode contacts and the movable contacts becomes increasingly difficult. The difficulty is derived form shrinkage in the molding process and expansion or contraction during use. For example, large sized covering members having a relatively large length of 20 to 50 cm or even larger are used in the preparation of large sized keyboard input units or push buttom input units for operation boards of, e.g., electric typewriters or electric cash registers. In the manufacture of such large sized covering members, shrinkage in the molding process and expansion or contraction during use can be as large as several percent relative to the size of the metal mold to cause mismatching in the position which may induce poor performance of the instrument. Taking account of the above, a measure for overcoming the difficulty in the positional matching is to use a metal mold having cavities with a marginal allowance based on the estimation of the molding shrinkage and expansion or contraction during use. Nonetheless, the positional matching with precision is still considered to be a difficult matter since the degree of molding shrinkage varies depending on the nature of the material in addition to the size and shape of the covering member in a sheet form and, furthermore, as a result of a small difference in the molding temperature and the processing time even when the materials are the same and also due to variation in the quality of the material between different lots.
Manufacture of large sized covering members of keyboard is also accompanied by a disadvantages of progressively increasing cost for the preparation of the metal mold as compared with the metal molds for smaller-sized covering members having dimensions, of, for example, 5 cm by 10 cm and also for a correspondingly large compression molding machine.
In addition, another drawback is involved in the prior art method of manufacture of covering members having differently colored push button elements to comply with the increasing demand in recent years in which simultaneous putting of an unvulcanized insulating rubber base with the conforming color in the cavities of the mold followed by vulcanization to cause mingling of the colorants on the boundary portions. Therefore, color arrangement of the molded products sometimes deviates from the intended design to decrease the adaptability to small-scale production of different kinds of products due to the marked decrease in the yield of acceptable products with an accordingly increasing cost.